6.7 地球形成和生命早期演变
Section outline
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Fiery Beginnings
::烟火开端This fiery hot lava flow shows what Earth's surface might have looked like shortly after the planet first formed about 4.5 to 4.6 billion years ago. Instead of rivers of , rivers of molten rock flowed over its surface. Life as we know it could not have survived in such a place. How did this inferno of a planet become today’s Earth, covered with water and teeming with life? The long and incredible story of Earth and its living things begins with the planet's formation.
::这一热热熔岩流显示了地球表面在45亿到46亿年前最初形成地球后不久的表情。 熔岩的河流流过地球表面,而不是它的河流。 我们所知道的生命不可能在这样的地方生存下来。 这个星球的地狱怎么会变成今天的地球,被水覆盖,充满生命?地球及其生物的漫长而令人难以置信的故事始于地球的形成。Earth Forms from Stardust
::来自星尘的地球表Our solar system began as a rotating disk of stardust. Then, a nearby star exploded and sent a shock wave through the disk, increasing its rate of spin. As a result, most of the mass became concentrated in the middle of the disk, forming the sun. Smaller concentrations of mass rotating around the center formed Earth and the other planets.
::我们的太阳系始于星尘的旋转磁盘。 随后,附近的一颗恒星爆炸,在磁盘中发出冲击波,加速了旋转速度。 结果,大部分质量集中在磁盘中间,形成太阳。在中心周围的较小质量浓度旋转形成了地球和其他行星。At first, Earth was molten and lacked an atmosphere and oceans. It may have looked like the volcanic image above. Gradually, the planet cooled and formed a solid crust. As the planet continued to cool, volcanoes released gases from under the crust, and the gases eventually accumulated to form an atmosphere. The early atmosphere contained ammonia, methane, water vapor, and carbon dioxide, but only a trace of oxygen. As the atmosphere became denser, clouds formed and rain fell. Water from rain (and perhaps also from comets and asteroids that stuck Earth) eventually formed oceans. The ancient atmosphere and oceans — represented by the picture — would be toxic to today’s life, but they set the stage for life to begin.
::起初,地球被熔化,没有大气层和海洋。它可能看起来象上面的火山图像。逐渐地,地球冷却并形成一个坚固的地壳。随着地球继续冷却,火山从地壳下释放气体,气体最终会聚集到大气中。早期的大气中含有氨、甲烷、水蒸气和二氧化碳,但只有氧气的微量。随着大气变稠,云层形成,雨水下降。雨水(也许还有彗星和悬浮在地球上的小行星)的水最终形成海洋。古代大气和海洋 — — 以图片为代表 — — 将对今天的生命有毒,但它们为生命的开始创造了舞台。Ancient Earth. This is how ancient Earth may have looked after its atmosphere and oceans formed.
::古代地球。这就是古代地球如何 保护其大气层和海洋形成的方式。Earliest Biochemical Molecules
::最强生化分子All living things consist of biochemical (organic) molecules. It is likely that biochemical molecules evolved before , perhaps as long as four billion years ago. How did these building blocks of life first form?
::所有生物都由生化(有机)分子组成。 生化(有机)分子很可能在40亿年前,也许在40亿年前就演变了。 这些生命的构件是如何形成的?One hypothesis is that lightning sparked chemical reactions in Earth’s early atmosphere, creating a “soup” of organic molecules from inorganic chemicals. In 1953, scientists Stanley Miller and Harold Urey used their imaginations to test this hypothesis. They created a simulation to see if organic molecules could arise in this way. The setup for their experiment is shown in the figure The two scientists started with a mixture of gases assumed to represent Earth’s early atmosphere. Then, they passed sparks through the gases to simulate lightning. Within a week, several simple organic molecules had formed.
::一种假设是闪电在地球早期大气中引发化学反应,制造了无机化学物有机分子的“烟雾 ” 。 1953年,科学家斯坦利·米勒和哈罗德·乌雷利用想象力测试了这一假设。 他们创造了一个模拟,以观察有机分子能否以这种方式出现。 其实验的设置在图中显示,两位科学家首先使用一种气体混合物,假定它们代表了地球早期的大气。 然后,他们将气体的火花传递到模拟闪电上。 一周内,几个简单的有机分子形成。Miller and Urey’s Experiment. Miller and Urey demonstrated that organic molecules could form under simulated conditions on early Earth. What assumptions were their simulation based upon?
::米勒和Urey的实验。 米勒和Urey证明有机分子可以在早期地球模拟条件下形成。 他们的模拟基于什么假设?Recently, the findings of Miller and Urey have been called into question because of discrepancies between their simulated atmosphere and the actual composition of Earth's early atmosphere. Other ideas regarding the formation of the have since been proposed. One hypothesis is that active volcanoes on early Earth provided the necessary materials for to form. However, there still is no definitive answer to the question of how organic molecules evolved. Whatever the process, it was probably spontaneous. Most likely, elements came together randomly to form small compounds, and small compounds reacted with other elements and small compounds to form larger compounds.
::最近,由于模拟大气层与地球早期大气层实际构成之间的差异,米勒和Urey的发现受到质疑。后来又提出了关于形成这种大气层的其他想法。一种假设是,早期地球上活火山提供了形成所需的材料。然而,对于有机分子如何演变的问题,仍然没有确切的答案。不管这一过程如何演变,它可能是自发的。最有可能的是,元素随机聚集在一起形成小化合物,小化合物与其他元素和小化合物一起形成更大的化合物。Another unresolved question is: which biochemical molecule evolved first? At various times, or have been hypothesized to be the first biochemical molecules. Living things need biochemical molecules to store genetic information and to carry out the chemical work of cells. Modern organisms use DNA to store genetic information and proteins to catalyze chemical reactions. DNA encodes proteins, and proteins are needed for life processes, so each of these two types of organic molecules needs the other for its own existence. How could either of these two molecules have evolved before the other? Did some other organic molecule evolve first, instead of DNA or proteins?
::另一个尚未解决的问题是:哪些生化分子首先进化?在不同的时间,或者被假定为第一个生化分子。生命需要生化分子来储存遗传信息和进行细胞的化学工作。现代生物利用DNA来储存基因信息和蛋白来催化化学反应。DNA编码蛋白和蛋白是生命过程所需要的,因此这两种类型的有机分子都需要另一个。这两种分子中,每一种都为自己的生存而存在。这两种分子中哪一种是先进的?其他的有机分子是先进的,而不是DNA或蛋白质的?Some scientists speculate that may have been the first organic molecule to evolve. They hypothesize that early life was based solely on RNA, and that DNA and proteins evolved later. This is called the RNA world hypothesis . Why RNA? It can encode genetic instructions (like DNA), and some RNA molecules can carry out chemical reactions (like proteins). Other evidence also suggests that RNA may be the most ancient of the organic molecules.
::一些科学家推测,这可能是第一个进化的有机分子。他们假设早期生命完全以RNA为基础,而DNA和蛋白质则后来演变。这被称为RNA世界假设。为什么RNA?它可以编码基因指令(如DNA),而一些RNA分子也可以进行化学反应(如蛋白质 ) 。 其他证据表明,RNA可能是最古老的有机分子。The Earliest Cells
::最深的细胞How the earliest cells evolved is also not known for certain, but scientists think that membranes grew around biochemical molecules. The membranes prevented the molecules from reacting with other molecules, so they did not form new compounds. In this way, the biochemical molecules persisted, and the first cells may have formed. The figure shows a hypothetical .
::最早的细胞是如何进化的也不为人所知,但科学家们认为膜是围绕生化分子生长的。 膜阻止了分子与其他分子发生反应,因此它们没有形成新的化合物。 这样,生化分子就继续存在,而第一批细胞可能已经形成。 图表显示了一种假设。Hypothetical First Cell. The earliest cells may have consisted of little more than RNA inside a lipid membrane.
::最早的细胞可能只是脂质膜内的RNA。No doubt there were many early cells of this type. However, scientists think that only one early cell eventually gave rise to all subsequent life on Earth. That one cell is called the Last Universal Common Ancestor (LUCA). It probably existed around 3.5 billion years ago. LUCA was one of the earliest prokaryotic cells . Therefore, it would have lacked a and other membrane-bound .
::毫无疑问,这种类型的早期细胞很多。然而,科学家认为只有早期细胞最终导致地球上所有随后的生命。一个细胞被称为“最后世界共同祖先 ” ( LUCA ) , 可能35亿年前就存在了。 LUCA是最早的人工细胞之一,因此,它缺少一种和其他的膜。Photosynthesis and Cellular Respiration
::光合成和细胞呼吸The earliest cells were probably heterotrophs , meaning that they could not produce their own energy . Most likely, they got their energy from other molecules in the organic “soup.” However, by about three billion years ago, a new way of obtaining energy evolved. This new way was . Through photosynthesis, organisms could use sunlight to make food from carbon dioxide and water. These organisms were the first autotrophs . They provided food for themselves, and for other organisms that began to consume them.
::最早的细胞可能是血化细胞,这意味着它们无法生产自己的能量。 最有可能的是,它们从有机“汤”中的其他分子中获得了能量。 然而,在大约30亿年前,一种获取能量的新方式发生了演变。这一新的方式就是光合作用,生物可以通过阳光从二氧化碳和水中提取食物。这些生物是第一个自发的。它们为自己和开始消费它们的其他生物提供了食物。After photosynthesis evolved, oxygen started to accumulate in the atmosphere. This has been dubbed the “oxygen catastrophe.” Why? Oxygen was toxic to most early cells because they had evolved in its absence. As a result, many of them died out. The few that survived evolved a new way to take advantage of the oxygen. This second major adaptation was aerobic . It allowed cells to use oxygen to get more energy from organic molecules.
::光合作用进化后,氧开始在大气中积聚。这被称为“氧气灾难 ” 。 为什么?氧对大多数早期细胞有毒,因为它们是在没有氧气的情况下进化的。结果,其中许多细胞死亡。幸存下来的少数细胞形成了一种利用氧气的新方法。第二大适应是有氧的。它允许细胞使用氧从有机分子中获取更多的能量。Evolution of Eukaryotes
::Eukaryotes进化The first eukaryotic cells probably evolved about two billion years ago. This is explained by endosymbiotic theory , which is illustrated in the figure . According to this theory , endosymbiosis came about when large prokaryotic cells engulfed small prokaryotic cells. The small cells were not digested by the large cells. Instead, they lived within the large cells and evolved into cell organelles.
::最初的雌激素细胞大概是在20亿年前进化的。这是由内共生理论解释的,图中说明了这一点。根据这个理论,在大型蛋白细胞吞噬小蛋白细胞时,就出现了内共生。小细胞没有被大细胞消化。相反,小细胞生活在大细胞中,演变成细胞器官。From Independent Cell to Organelle. The endosymbiotic theory explains how eukaryotic cells evolved.
::从独立细胞到Organelle。内生理论解释了 尿道细胞是如何进化的。The large and small cells formed a symbiotic relationship in which both cells benefited. Some of the small cells were able to break down the large cell’s wastes for energy. They supplied energy not only to themselves, but also to the large cell. They became the of eukaryotic cells. Mitochondria are the organelles where two of the three stages of aerobic cellular respiration take place. Other small cells were able to use sunlight to make food, which they shared with the large cell. They became the photosynthetic organelles (called chloroplasts) of autotrophic eukaryotic cells.
::大型和小型的细胞形成了一种共生关系,使两个细胞都受益。有些小细胞能够将大细胞的废料分解为能量。它们不仅为自己,而且为大细胞提供能量。它们成为了电子细胞。Mitochondria是三阶段有氧细胞呼吸的两个阶段的有机体。其他小细胞能够利用阳光制造食物,它们与大细胞分享这些食物。它们成为了自养细胞的光合有机体(称为叶绿板 ) 。With their specialized organelles, eukaryotic cells were powerful and efficient. They would go on to evolve additional major adaptations. These adaptations would eventually include , cell specialization , and multicellularity. Over time, eukaryotic cells would evolve into the plants, , and other eukaryotes we know today.
::随着它们的专业化器官,电子化细胞既强大又高效。 它们将继续发展更多的重大适应。 这些适应最终将包括细胞专业化和多细胞性。 随着时间的推移,电子化细胞会演变成植物,以及我们今天所认识的其他电子化体。Summary
::摘要-
Earth formed 4.5-4.6 billion years ago. At first, Earth was molten and lacked an atmosphere and oceans. Gradually, the atmosphere formed, followed by the oceans.
::地球在45—46亿年前形成。最初,地球被熔化,没有大气层和海洋。逐渐形成,然后是海洋。 -
The first organic molecules probably formed by four billion years ago. This may have happened when lightning sparked chemical reactions in Earth’s early atmosphere. RNA may have been the first organic molecule to form.
It was also
the basis of early life.
::最早的有机分子可能是40亿年前形成的。 这可能是当闪电在地球早期大气中引发化学反应时发生的。 RNA可能是最早形成的有机分子。 这也是早期生命的基础。 -
The first cells consisted of little more than an organic molecule such as RNA inside a lipid membrane. One cell — called the last universal common ancestor (LUCA) — gave rise to all subsequent life on Earth.
::第一批细胞只包括一种有机分子,如脂质膜内的RNA, 一种叫作最后一个普遍共有祖先的细胞(LUCA),在地球上产生了所有随后的生命。 -
Photosynthesis evolved by three billion years ago, and released oxygen into the atmosphere.
Aerobic c
ellular respiration — which made use of the oxygen — evolved after that.
::光合作用在30亿年前形成,释放氧气到大气中。 之后,利用氧气的有氧细胞呼吸也随之演变。 -
Eukaryotic cells probably evolved about two billion years ago. Their evolution is explained by endosymbiotic theory. Eukaryotic cells would go on to evolve into the diversity of eukaryotes we know today.
::神经细胞的进化可能发生在20亿年前。 它们的进化是由内生理论解释的。 神经细胞会继续演变成我们今天所知道的电子细胞的多样性。
Review
::回顾1. Summarize how Earth formed and how its atmosphere and oceans developed.
::1. 总结地球如何形成及其大气层和海洋如何发展。2. Describe Miller and Urey’s experiment. What did it demonstrate?
::2. 描述米勒和Urey的实验,它展示了什么?3. State the RNA world hypothesis.
::3. 说明RNA世界假设。4. What was LUCA? What were its characteristics?
::4. LUCA是什么?其特征是什么?5. Explain why aerobic cellular respiration could evolve only after photosynthesis had evolved.
::5. 解释为什么有氧细胞呼吸只有在光合作用演变后才能演变。6. Why do you think that a barrier against the outside environment (such as the lipid membrane that probably formed around early cells) is important to the origins of life?
::6. 为什么你认为对外部环境设置障碍(如可能在早期细胞周围形成的脂质膜)对生命起源很重要?7. True or False: Biochemical molecules probably evolved before cells.
::7. 真实或假:生化分子可能先于细胞进化。8. True or False: There was only one early cell that formed, and all life is descended from that cell.
::8. 真实或假:只有一个早期细胞形成,所有生命都是从细胞中下来的。9. True or False: Many early cells died when oxygen became prevalent in the atmosphere.
::9. 真实或假:许多早期细胞在空气中氧气泛滥时死亡。10. Which gas was probably not very abundant in the initial atmosphere of Earth?
::10. 在地球最初的大气层中,哪些气体可能并不十分丰富?a. water vapor
::a. 水蒸气b. oxygen
::b. 氧氧气c. methane
::c. 甲烷d. carbon dioxide
::d. 二氧化碳11. The earliest cells were probably...
::11. 最早的细胞可能是...a. photosynthetic
::a. 光合作用b. eukaryotes
::b. eukaryotes( eukaryotes)c. autotrophs
::c. 自养d. heterotrophs
::d. 血红蛋白12. What is endosymbiotic theory? Name two organelles that are probably the result of endosymbiosis.
::12. 内生生物理论是什么?13. Of the following choices, which is the most recent ancestor to humans?
::13. 在以下选择中,哪一个是人类最近的祖先?a. single-celled eukaryotes
::a. 单变型eukaryotesb. single-celled prokaryotes
::b. 单细胞原圆柱形c. large prokaryotes
::c. 大型丙烯烯酸酯d. LUCA
::d. 土地利用的变化和林业Explore More
::探索更多Check out this video about the formation of Earth:
::校对:PortnoyImagine you swallowed a small bird and suddenly gained the ability to fly. Well, throughout the history of life (and specifically during the evolution of complex eukaryotic cells), things like this happened all the time. Check out this video about endosymbiosis, a type of symbiosis in which one symbiotic organism lives inside another:
::想象一下你吞下一只小鸟,突然获得飞行能力。在整个生命史上(特别是在复杂的水晶细胞进化过程中),类似的事情经常发生。看看这个关于内生生物的视频,这是一种共生生物,其中有一个共生生物存在于另一个生物体内: -
Earth formed 4.5-4.6 billion years ago. At first, Earth was molten and lacked an atmosphere and oceans. Gradually, the atmosphere formed, followed by the oceans.
